Atomic Theory and the Atomic Model

Objectives Describe evidence that Greek philosophers used to develop the idea of atoms. Distinguish between a scientific theory and a law Compare and contrast Dalton’s atomic theory with modern atomic theory Explain how Dalton’s atomic theory explained the laws of conservation of mass, definite proportions, and multiple proportions

Early Greek Philosophers Democritus –First proposed the concept of tiny particles called atomos (atoms) –“Uncuttable” particles –Believed that atoms: –Moved through empty space –Cannot be created or destroyed Aristotle –Did not agree with Democritus –Did not believe that “empty space” could exist

Law vs Theory A law describes observed behavior A theory attempts to explain observed behavior –“Models”

Dalton’s Atomic Theory (1803) (p. 104) All matter composed of very small particles called atoms Atoms of a given element are identical in size, mass, and other properties; atoms of different elements are different from atoms of any other element

Dalton’s Atomic Theory (p. 104) Atoms cannot be created, subdivided, or destroyed –Atoms are indestructible Atoms of different elements can combine in simple, whole-number ratios to form compounds In chemical reactions atoms are combined, separated, or rearranged but not changed into other atoms

Three Observed Laws (Late 18 th Century) Law of Conservation of Mass Law of Definite Proportions Law of Multiple Proportions

Law of Conservation of Mass Matter cannot be created nor destroyed in ordinary chemical or physical reactions.

Conservation of Mass Atoms don’t “appear” or “disappear” Atoms only rearrange –Dalton’s model Reaction products have the same numbers and types of atoms as reactants

Conservation of Mass (Explained by Dalton’s Theory) B 1 amu A 3 amu B 1 amu A 3 amu B 1 amu B 1 amu + A 3 amu A 3 amu  A 2 (g) + B 2 (g)  2 AB (g) 6 amu + 2 amu  4 amu + 4 amu

Law of Definite Proportions Any chemical compound will contain the same elements in the same proportions by mass, whatever the size or the source of the sample

Definite Proportions Atoms don’t “split” –Dalton’s Model A compound always has the same atoms in the same proportion Additional molecules of compound have the same atoms in the same proportions

Definite Proportions (Explained by Dalton’s Theory) B 1 amu A 3 amu A:B = 3amu:1amu B 1 amu A 3 amu B 1 amu A 3 amu A:B = 6amu:2amu

Law of Multiple Proportions Elements combine in simple whole number mass ratios to form compounds

CompoundA (g)B (g)A/B / / /1 Ratios of A:B are simple whole numbers

B 1 amu A 3 amu A:B = 3:1 B 1 amu A 3 amu A 3 amu Atoms cannot subdivide during chemical reactions

B 1 amu A 3 amu A 3 amu A:B = 6:1

B 1 amu A 3 amu A 3 amu A 3 amu A:B = 9:1

Atomic Structure The identity and arrangement of smaller particles within atoms

Cathode Ray Experiments J.J. Thomson Resulted in the first major change to Dalton’s theory He discovered the first sub-atomic particle –Atom was not indivisible

The Cathode Ray Tube If the tube was placed in an electric field the beam was attracted to the positive plate

Confirmed existence of electrons Implied existence of protons Importance of Thomson’s Cathode Ray Experiments

Models of the Atom DALTON Indivisible THOMSON “Plum Pudding”

Modern Atomic Theory All matter is made up of very small particles called atoms Atoms of the same element have the same chemical properties; atoms of different elements have different chemical properties Any natural sample of an element will have a characteristic average mass, different from any other element

Modern Atomic Theory Compounds are formed when atoms of two or more elements unite, each losing its individual properties in the combination Atoms are not subdivided in ordinary physical or chemical reactions

Discovery of the Nucleus

Results of foil experiment if Plum Pudding model had been correct.

Actual results

The Nucleus Positively charged –Contains protons and neutrons Very dense Central portion of the atom –Contains most of the atom’s mass, but... –Occupies only a tiny fraction of its volume

Particles in the Atom ParticleMassCharge Proton1 amu+1 Neutron1 amu0 Electron1/1800 amu–1

Nuclear Forces Short-range forces that hold the nucleus together *Proton-proton *Proton-neutron *Neutron-neutron

Rutherford’s Model of the Atom The nucleus is positively charged and very dense Most of the atom is empty space –Electrons move in the empty space